The basics: procedural vs. substantive law
Procedural law is about how the law gets passed and enforced: who has jurisdiction over whom, and what coercive processes they may use to bring suspected lawbreakers to justice. The famous Miranda lines "you have the right to remain silent...." generally uttered in the U.S. when you are arrested are a species of U.S. federal procedural law. Procedure usually starts in a given case with a great deal of uncertainty and tries to reduce that uncertainty by fairly gathering and evaluating evidence, interpreting the law, and applying those facts to the law to reach legal conclusions.
Substantive law involves every law that is not procedural: it is what we normally discuss when talking about law or politics, namely the laws defining and restricting rights and duties for their own sake, not primarily for the sake of enforcing other laws.
Thus for example modern property, contract, tort, family, and criminal law are substantive legal areas, as are environmental, workplace, traffic, and most other regulations. On the other hand, the laws defining who may sue whom and where, and what does and does not constitute proper arrest, interrogation, and search of criminal suspects, are procedural laws. Historically, just to confuse things a bit, property rights sometimes included rights of coercive procedure, for example the lord who had jurisdiction over his unfree tenants. This made property law in some cases part of the procedural law as well as a substantive law of economic property.
Computer protocols work in layers: wires carry bits of information, and bits of information carry text, pictures, and so on. The raw bits of information are a lower level protocol that carries the text and pictures in a higher level protocol. Language works like this too: at the lowest level, paper has letters written on it. Letters are a lower-level protocol that carries words in a higher level protocol. You can think of the distinction between substantive and procedural law in the same way: the procedural layer is a lower layer that "carries" the substantive law by specifying how it is to be enforced.
We can also think of government and government-like entities as lower levels of the legal protocol. Indeed, it is very useful to study political structures alongside procedural law. Think of coercive entities like police and courts as the paper and pencil, procedural law as the letters, and substantive law as the words and sentences we want to make out of these raw materials.